Control of the Reaction Force When Generating Angular Impulse

Abstract

Introduction: During the take-off phase of a backward rotating dive, the mechanical objective is to generate adequate linear and angular impulse to safely clear the platform and generate the angular momentum required to complete the dive prior to water entry. The moment created by the vertical reaction force about the total body center of mass (TBCM) contributes to backward rotation, whereas the moment created by the horizontal reaction force impedes backward rotation (Miller et al., 1989). The purpose of this paper was to determine how divers generate and control a reaction force acting a distance from the TBCM. We hypothesized that divers generating relatively large angular impulse would generate less horizontal impulse than divers generating smaller angular impulse during the take-off phase. Methods: Eight US National divers (female = 3, male = 5) performed a series of platform dive take-offs from a force plate into a foam landing pit as part of a training session supervised by their coach. For this study, the backward rotating dives were analyzed. Reaction forces (600 Hz, Kistler, 0.6 × 0.9m) and sagittal plane kinematics (60 fps) during take-off were recorded simultaneously. Details of data conditioning and processing have been described in the previous research (Mathiyakom et al., 1998). According to the vertical reaction force-time curve, the take-off phase was divided into three phases: Press (Initial increased of reaction force to the first peak), Tip (the first peak to local minimum), and Push (local minimum to plate departure). Angular impulse was calculated as integration of the moment of the vertical and horizontal force about the TBCM during each phase. Results: As hypothesized, divers generating relatively large net angular impulse during the take-off phase generated less horizontal impulse and less angular impulse generated by the horizontal reaction force (Figure 1). Divers generating relatively large angular impulse generated greater impulse during the push phase than divers with relatively small angular impulse. The correlation between the net angular impulse and the impulse during the press, tip, and push phases were 0.09, 0.84 (p < 0.5) and 0.96 (p < 0.5), respectively.Figure: Net Angular Impulse (Nms/Kg)Discussion: Generation of angular impulse during the take-of phase of a backward dive requires the diver to generate a reaction force anterior to their TBCM. Between subject differences in the horizontal reaction force component also suggests that direction of the external reaction force generated will contribute to the magnitude of the net angular impulse generated, particularly during the push phase. Differences in external reaction force generation may also have implications on the contribution of lower extremity joints to control and generation of the reaction force. Acknowledgements: This project was funded in part by US diving, Intel, and US Olympic Commiteee.